The invention relates to image sensors and random access memory. More specifically, the invention relates to a CMOS image sensor that is integrated with random access memory.
Charge coupled devices (xe2x80x9cCCDsxe2x80x9d) and complimentary metal oxide semiconductor (xe2x80x9cCMOSxe2x80x9d) imaging sensors are presently being used in consumer electronic products such as digital cameras, PC video cameras, laptop computers, cellular phones and toys. They are also being used in high-quality digital imaging applications such as surveillance and security cameras, and in biometric measurement applications such as fingerprint-recognition systems.
CMOS imaging devices offer a number of advantages over CCDs. CMOS imaging devices consume much less power than similar CCDs. This advantage is particularly important for consumer electronic products.
CMOS components are fabricated in mainstream silicon foundries. Because CMOS is an established technology that is used for many different types of integrated circuits, it has the advantage of manufacturing economies of scale that drive down defect density and production costs. Higher yields and less susceptibility to defects make CMOS a lower cost technology for image sensors than CCD. Fewer parts, a smaller form factor, and higher reliability in the end products mean cost savings to systems manufacturers.
CMOS imaging devices tend to produce better images than CCDs. CCDs rely on a process that can leak charge to adjacent pixels when the CCD register overflows; thus bright lights xe2x80x9cbloomxe2x80x9d and cause unwanted streaks in captured images. CMOS imaging devices are inherently less sensitive to this effect. In addition, smearxe2x80x94caused by charge transfer in the CCD under illuminationxe2x80x94is non-existent with CMOS imaging devices.
CMOS imaging devices allow for higher system integration. With CMOS, signal processing can be integrated directly with the imaging device on a single chip. On-chip analog-to-digital (xe2x80x9cA/Dxe2x80x9d) conversion facilitates driving high-speed signals off-chip. Once converted to digital, sensor signals are less sensitive to pickup and crosstalk. Timing control can be integrated onto the sensor chip. On-chip digital signal processing functions can also be performed, from color encoding, image stabilization and compression to motion tracking, video conferencing and wireless control. In addition, standard camera functions such as Automatic Gain Control, color balance, and automatic exposure and focus control can be performed on the sensor chip.
It would be desirable to integrate random access memory (xe2x80x9cRAMxe2x80x9d) onto the sensor chip. However, there are certain problems to overcome. One problem is voltage sag. A typical CMOS active pixel sensor includes multiple stages of charge amplifiers. Each stage reduces strength of the sensor signal.
Another problem is converting the CMOS sensor signals to digital prior to storage in the RAM. The amount of silicon area taken up by A/D converters can be quite substantial. Integrating conventional A/D converters with each pixel would greatly increase the size and cost of the sensor chip, especially if high resolution is desired. Reducing the number of AND converters by, for example, using only one A/D converter per row of pixels would reduce chip size and cost; however, it could create fixed pattern noise in the image. Human eyes are very sensitive to detecting fixed pattern noise.
These problems are overcome by the present invention. According to one aspect of the present invention, a CMOS imaging device includes a code generator and a plurality of pixels. Each pixel includes a CMOS sensor, a comparator and random access memory. At the beginning of a capture mode, the code generator is started. Within each pixel, the comparator compares an output of the CMOS sensor to a threshold. When the output crosses the threshold, a code word from the code generator is stored in the random access memory. Thus, raw pixel data is represented by digital code words. The code words can be converted afterwards to raw pixel data.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.